Prevention of undercutting of latent xeroradiographic images



Aug. 13, 1957 E. H. LEHMANN ,802,949 PREVENTION OF UNDERCUTTING OF LATENT XERORADIOGRAPHIC IMAGES Filed Dec. 6, 1954 INVENTOR ATTORNEY PREVENTION OF UNDERCUTTING OF LATENT XERORADIOGRAPHIC Ill [AGES Ernest Henry Lehmann, Rochester, N. Y., assignor to The Haloid Company, Rochester, N. Y., a corporation of New York Application December 6, 1954, Serial No. 473,097

6 Claims. (Cl. 25065) This invention relates to xeroradiography and more particularly to a method and arrangement for eliminating undercutting of latent xeroradiographic images. The undercutting of latent images of radiographed objects means that the images reproduced appear smaller than their actual size because the edges of the image disappear or are much fainter than the remainder of the image of the radiographed object in the radiographic record.

In xeroradiography the plate or element which is exposed to X-ray or gamma ray or other penetrating radiation usually comprises a conductive metallic backing sheet having a photoconductive insulating layer or coating, for example, of vitreous or amorphous selenium on one surface. It is conventional to cover or protect the coating from light by the use of a so-called dark slide which is somewhat spaced from the surface of the coating. The plate or element is sensitized by applying an electrostatic charge to the coating, and then the sensitized plate or element is exposed to-penetrating radiation with the object to be radiographed appropriately interposed between the radiation source and the plate. Under the influence of the radiation from the source which readily passes through the dark slide, the coating becomes electrically conductive, permitting the electrostatic charge thereon to be selectively dissipated in the portions reached by the penetrating beams, less dissipation occurring in those portions of the coating that are shaded by the object being radiographed. In this manner an electrostatic latent image of the radiographed object is formed on the coating. This image may then be developed, for example, with an electrostatic material which clings to the electrically charged portions of the latent image on the coating.

When a radiographic exposure is made in the manner described, the latent image produced on the element or plate is frequently smaller than its actual size, or in some instances is materially weakened around its edges so that it may appear smaller in size. This effect is not created necessarily by a physical reduction in size of all dimensions, but is merely due to a disappearance of the latent image at its edges, i. e., these edges are undercut.

One of the causes of such undercutting is the scattering of secondary X-rays from the radiographed object. A second cause is known experimentally to be the result of air ionization produced in the air space between the dark slide and the surface of the photoconductive coating. This air ionization is caused by primary X-rays, secondary X- rays and secondary high speed electrons. This invention concerns itself as a primary object with methods and arrangements for the substantial elimination of ionization-induced undercutting.

Through experiment it is known that after exposure of the plate or element to penetrating radiation the ionization of the air in the space directly above the plate or element causes an uncontrollable discharge, particularly around the peripheries of the latent image formed on the plate, with consequent fading in sharpness of the outlines of the radiographed image or complete disappear- Patented Aug. 13, 1957 ance of these outlines resulting in the above-mentioned undercutting. There are several ways of eliminating the undesired ionization-caused discharge. For example, it may be partially controlled by suitably filtering out the soft component of the penetrating Xaray beam which causes the ionization. Another way of eliminating the ionization is by exposing the surface of the charged xeroradiographic plate or element to a possibly non-ionizing or self-quenching gas such as a Freon (fluorinated hydrocarbon) or sulfur hexafluorine. A third practice is to place a thin insulating medium in direct contact with the sensitized surface of the plate or element whose contact with such surface is intimate and prevents access of any ionized gas to the charged surface. A requirement of such insulating material is that it must not materially decrease the intensity of the penetrating radiation reaching the sensitized surface. The easy application of the insulating medium is one of its advantages.

It is found that the practice of the third-mentioned solution also has a number of other advantages besides that just mentioned. Its practice is in fact a universal solution for all required degrees of intensity of exposure to penetrating radiation. The first-named solution utilizing filters on the other hand requires, due to exponential absorption, the use of increasingly thicker filters for greater intensity exposures. Such filters in addition may affect the contrast sensitivity as well as prolong the required exposure time. In practice, therefore, the prevention of air-ionization-caused undercutting by the use of insulating media in contact with the sensitized surface is found to be superior to the other solutions mentioned. The manner of utilization of such insulating media and their nature and their specific application to sensitized surfaces may be variegated.

In its broadest aspect the invention contemplates elimination of ion-caused undercutting by excluding ionized or ionizable gas such as air from the surface of the sensitized or photoconductive layer of the xeroradiographic plate and exposing the plate to the penetrating radiation during such exclusion.

The exclusion may be effected by evacuation of ionizable gas or prevention of access of such gas with the surface, or by elimination of ionizing radiation components from the penetrating radiation.

Objects and features of the present invention are to provide an improved xeroradiographic process, utilizing insulating or other appropriate media in conjunction with the sensitized plates or elements to substantially eliminate or materially reduce undercutting as caused by air ionization occurring during radiation exposure.

Further objects and features of the invention are the production of simple arrangements for excluding ionized air or gas, for example, by maintaining the xeroradiographic plate or element in surface contact with thin insulating media so as to prevent access of ionizing air to the sensitized surface of the plate or element.

Further objects and features of the invention will become apparent from the following specification and the accompanying drawings, wherein:

Figure 1 is a cross-sectional view of a xero-radiographic element or plate to whose sensitized surface a thin insulating medium has been applied for the purposes of practicing the present invention; and

Figure 2 is a similar view of an alternative arrangement for maintaining surface contact between a thin insulating medium in the form of a flat balloon and the sensitized surface of a xeroradiographic plate or element for the purposes of practicing the instant invention.

Referring to the figures in detail, and first to Figure l, a xeroradiographic member is shown being generally designated 10 and comprising a xeroradiographic plate or element 11 surrounded by and engaging a rigid frame 12 which surrounds and protects the four sides of the plate. In the preferred embodiment this frame is lighttight and is further provided wtih a dark screen or slide 13 of conventional material. The slide is mounted slideably in the frame 12, for example, within the grooves 14.

The xeroradiographic plate 11 consists generally of a conductive metallic backing plate 15, having on at least one of its surfaces a coating or layer 16 of an insulating photocondutive or vitreous form of selenium or other material of similar properties. This coating or layer 16, as is known, has the property of retaining in the dark an electrostatic charge applied to it as by friction or by a corona discharge, and of discharging or disispating its retained charge when it is exposed to light or to X-ray or gamma ray or other penetrating radiation. Photoconductive material as used in this specification, is meant to include either material which is normally non-conductive except when exposed to penetrating radiation, for example, of X-rays, or to materials rendered conductive either by such exposure or by exposure to light.

The dark shield or slide 13 is slideable outwardly of the frame 12. to uncover the selenium coating 16 so as to permit charging of the coating and also the application to the surface of the latter of a thin insulating medium 17. The slide 13 fits in light-sealing relationship in the grooved frame 12 to prevent undesired discharge of the charged coating 16 by unwarranted exposure to light.

Under usual xeroradiographic operating techniques heretofore practiced, where no electrical connection exists between the dark slide and the xeroradiographic plate or element 11, air ionization occurs in the air space 18 between the slide and the coating. Ion pairs produced in the air gap or air space 18 by passage therethrough of the penetrating radiation will have their negative ions attracted to areas of higher charge of the coating 16 and because of the existence of fringing fields (not shown) will partially discharge the plate 11, starting at the edges of the charged areas. As areas of the plate are discharged either by internal conductivity or by these ions, the positions of the fringing fields shift inwardly toward the middle of the still-charged portions. In consequence, undercutting is produced in the sense that the images are lost or so weakened as to be practically lost.

The essence of the instant invention lies in the discovery that this undercutting can be eliminated substantially by the prevention of direct access of the ions to the charged surface of the coating 16 of the plate or element 11 so that the negative ions in the air space 18 cannot effect a discharge at the boundaries between areas of higher and of lower charges in the surface of coating 16.

In putting into practice this discovery, in one embodiment of the invention a thin sheet 17 of insulating material, for example, of plastic insulating material, or of rubber, or of dry clean paper, is applied to the charged outer surface of the selenium coating 16 and into intimate contact with the latter. The intimate contact is essential in order to prevent access to the charged surface of the layer 16 of ionized air appearing in the space 18 between said surface and the dark slide 13, e. g. to prevent bubbles of gas that are trapped between the layer and the plate from locally discharging (undercutting) the image. The dark slide 13 is then mounted in the frame 12 and the image or object O that is to be radiographed is positioned on the dark slide. Thereafter, exposure is made by turning on the radiation penetrating source such as the X-ray tube T for the required period of exposure. On completion of the exposure the object is removed, the dark slide removed and the insulating film or sheet 17 separated from the selenium layer 16.

Care must be taken both in the application and in the removal of the insulating sheet 17 from the selenium layer 16 to prevent the formation of Lichtenburg figures whose presence would produce inaccuracies in the latent image on layer 16. These figures may be minimized or avoided by laying the thin layer 17 of rubber, plastic or paper on the surface of the selenium layer 16 without friction or rubbing and in similarly removing such materials. In addition, the insulating sheet 17 of film must be free of all charge prior to its application to the selenium surface. It may be rid or freed of its latent charges, for example, by exposure to X-ray, prior to its application to the selenium layer 16 or in other conventional ways.

The materials suitable for the insulating film or layer are insulating plastics such as cellulose acetate or cellophane sheeting or rubber or clean dry paper. The thickness of the film or layer 17 may vary in practical application. it must be a relatively thick layer, such that the mean free ion path in the material is short compared with the thickness. Other materials having an electrical resistance which is greater than the dark resistivity of the selenium layer 16 also would be suitable rather than purely insulative materials such as those mentioned. The advantages of such semi-conductive materials of high resistivity are that they would be self-discharging while at rest and when in contact with the selenium layer would not have a conductivity great enough to seriously afiect the dissimilar potential areas in the selenium surface 16 during and after exposure of the latter. With such materials any deterioration of the electrical image in the selenium layer would be gradual and not as serious as the present uneven decay or undercutting caused by air ionization.

There are other ways of applying a thin insulating layer into surface contact with the selenium layer 16. One such other way which is illustrated in Figure 2 is by the substitution for the thin layer 17 of Figure 1 of a fiat balloon 19 of rubber or other flexible inflatable insulating material. This balloon 19 preferably is either permanently inflated or else may be inflated as used. In this embodiment the balloon is dimensioned so that it may be readily compressed in the space 17 defined bebetween the dark slide. 13 and the exposed surface of the selenium layer 16. When the balloon 19 is located in this way and the dark slide 13 is in place, the resiliency of the balloon material when the latter is inflated causes it to be pressed into intimate contact with the exposed surface of the selenium layer 16 due to the action of the compressed air within the balloon. The intimate contact between the material of the balloon 19 and the surface of the layer 16 under the pressure of the compressed air excludes air from contact with the surface of the selenium layer 16 and thus prevents the access of undercutting causing ions to the surface of the selenium layer 16. While Figure 2 shows the utilization of the dark slide 13 to effect the compression of the balloon material against the surface of the layer 16, it is obvious that other mechanical expedients may be utilized to produce the same effect.

Either the layer 17 of Figure 1 or the balloon 19 of Figure 2 has substantially no material effect upon the penetration of the X or other type radiation and, therefore, neither of them is an impediment to proper exposure of the sensitized selenium layer 16. At the same time the intimate contact either of the layer 17 or ma terial balloon 19 with the selenium layer 16 precludes the access of ionized air to the selenium layer 16 and, as noted above, this prevents substantially all undercutting of the radiograph image on the layer 16.

It is a frequent practice to invert the xeroradiographic element 10 so that its plate 11 is uppermost and its selenium layer 16 is at the bottom and with the dark slide 13 lying below the layer 16. In such inverted position the exposure of the object occurs through the backing plate 15 rather than through the dark slide 13. An advantage of the inversion is that the test object being radiographed is as close as physically possible to the xeroradiographic plate 11, thus reducing any tendencies for distortion of the image to a minimum. As long as the insulating layer 17 or the material of balloon 19 remain in intimate contact with the surface of the selenium layer 16 any ionized air present in the space 18 ambient to the surface of layer 16 is prevented from coming into contact with the said surface of the selenium layer 16 and undercutting is thus prevented.

While specific embodiments and modes of practicing the invention have been described, variations within the scope of the appended claims are possible and are contemplated. There is no intention, therefore, of limitation to the exact details shown or described.

What is claimed is:

1. That improvement in xeroradiography of objects to eliminate undercutting of images of the radiographed object produced on a xeroradiographic plate comprising the application in intimate contact with the surface of a sensitized photoconductive layer of said plate of a thin film of substantially non-conductive material to preclude access to the said surface of ionized gas and exposing the xeroradiographic plate to penetrating radiation While maintaining said contact.

2. Apparatus for eliminating undercutting of the image of a radiographed object comprising a xeroradiographic element and means for preventing ionization discharge of the element during exposure of the latter to penetrating radiation.

graphic element including a chargeable photoconductive layer and a thin covering of material whose resistivity exceeds the dark resistivity of said layer applied in intimate contact with the surface of said layer.

4. Apparatus according to claim 3 wherein the covering is an insulative material.

5. Apparatus for eliminating undercutting of the image of a radiographed object comprising a xeroradiographic element including a photoconductive layer and a covering balloon whose material has a resistivity that exceeds the dark resistivity of said layer and means for effecting intimate contact between said layer and the material of said balloon to prevent access of ionized gas to the surface of said layer.

6. Apparatus according to claim 5 wherein the balloon is of thin rubber, and including a frame for supporting the plate and dark slide movable in the frame and spaced from the plate, said balloon being compressed in the space between said dark slide and said surface.

References Cited in the file of this patent UNITED STATES PATENTS Buck Aug. 18, 1925 Uhle et al. Aug. 28, 1951 ppa a s r eliminating r ut ng f th C. McMaster in Magazine Non-Destructive Testing, vol.

image of a radiographed object comprising a xeroradio- 10, No. 1, Summer Number 1951, pages 9 and 23. 

